Run Flask app as fixture

test_flask.py

from threading import Thread

import pytest
import requests
from flask import Flask


@pytest.fixture()
def app() -> Flask:
    host = 'localhost'
    port = 5000

    # Create app with shutdown hook
    app = Flask(__name__)
    app.env = 'development'
    app.testing = True

    @app.route('/shutdown')
    def shutdown():
        from flask import request
        request.environ['werkzeug.server.shutdown']()
        return 'OK', 200

    # Start server
    thread = Thread(target=app.run, daemon=True, kwargs=dict(host=host, port=port))
    thread.start()

    # Provide app for testing
    yield app

    # Shutdown server
    requests.get('http://localhost:5000/shutdown')

    thread.join()


def test_flask(app):
    @app.route('/ping')
    def ping():
        return 'PONG'

    requests.get('http://localhost:5000/ping').raise_for_status()

Since werkzeug.server.shutdown has been deprecated and removed, the above no longer works. However, if you only need the server to start once before running the tests (at the session level), then you can skip thread.join() because, when the tests finish, it will be the only thread left running and the Python process exits when only daemon threads are left. Details here: https://docs.python.org/3/library/threading.html#thread-objects

This is the code ended up with in my conftest.py:

@pytest.fixture(scope='session')
def app():
    app = create_app()

    thread = Thread(target=app.run, daemon=True, kwargs=dict(host='localhost', port=5000))
    thread.start()

    yield app


@pytest.fixture(scope='module')
def client(app):
    return app.test_client()

All the tests which need this full-fledged Flask server to be up and running must to use the client fixture which depends on the app fixture. For example:

def test_ping(client):
    response = client.get('http://localhost:5000/ping')

    assert response.status_code == 200